The Discovery of Insect Germ Plasm
The germinal cytoplasm of insects is different from any other cytoplasm in the egg. Hegner (1911) found that when he removed or destroyed this region of beetle eggs before pole cell formation had occurred, the resulting embryos had no germ cells and were sterile. This pole plasm contained the determinants of the germ cells. Geigy (1931) showed that irradiating Drosophila egg pole plasm with ultraviolet light produced sterile flies; Okada and co-workers (1974) extended this line of experimentation, by showing that the addition of pole plasm from unirradiated donor embryos can cure the sterility of irradiated eggs (Figure 1). No other part of the cytoplasm could accomplish this reversal of sterility.
The importance of insect germ plasm had been shown by critical experiments before the age of molecular biology. One of these investigations concerned chromosome diminution in the midge Wachtiella persicariae. In this species, most nuclei lose 32 of their original 40 chromosomes! However, two undiminished nuclei are found at the posterior pole of the egg and do not divide for a period of time. These two nuclei eventually give rise to the germ cells. When nuclei are prevented, by a ligature, from migrating into the posterior pole region, every nucleus undergoes diminution, and the resulting midge is sterile. When the ligature is loosened, and a diminished nucleus enters the posterior pole, germ cells are made but never differentiate into functional gametes (Geyer-Duszynska 1959). Kunz and co-workers (1970) have shown that the eliminated chromatin contains genes that are active during germ cell production.
The autonomy of this cytoplasmic region and its ability to determine any Drosophila nucleus was shown in 1974 by the ingenious experiments of Karl Illmensee and Anthony Mahowald. In these experiments (outlined in Figure 2), an incredibly small amount (5–100 picoliters) of anucleate pole plasm was transferred from wild-type Drosophila eggs into the anterior pole of genetically marked eggs prior to their cellularization. These donor eggs carried the chromosomal mutations multiple, wing, hair, and ebony. After the cellular blastoderm had formed, the cells in the anterior pole of the recipient embryo resembled normal posterior pole cells, having incorporated the polar granules and developed a typical pole cell morphology. To test whether or not these cells had become functioning germ cell precursors, Mahowald and Illmensee transplanted these modified anterior cells into the posterior region of cleaving embryos containing their own genetically marked pole cells. (The reason for doing this was so that these cells might be incorporated into the developing gonads. There is evidence from other organisms that the pole plasm also contains determinants for the proper migration of germ cells.) These new host embryos were marked by different mutations (recessives yellow, white, and singed). When these embryos developed, they all became flies bearing the mutations yellow, white, and singed. These flies were mated to other flies carrying the same mutations. In most cases (88/92), these matings produced individuals identical to both parents. In four cases, however, wild-type progeny emerged, indicating that some of the germ cells in these flies derived from the transplanted cells; the germ plasm from one embryo was able to cause the anterior nuclei of another embryo to develop into functioning germ cells! This technique has also proved useful in showing the time of the germ cell determinant's localization. Illmensee and his colleagues (1976) found that this determinant is able to function before fertilization and becomes localized in the developing oocyte at about the same time that the yolk reaches the posterior end of the egg.
Geigy, R. 1931. Action de liultra-violet sur le pole germinal dans lioeuf de Drosophila melanogaster (castration et mutabilitE). Rev. Suisse Zool. 38: 187-288.
Geyer-Duszynska, I. 1959. Experimental research on chromosome diminution in Cecidomiidae (Diptera). J. Exp. Zool. 141: 381-441.
Hegner, R. W. 1911. Experiments with chrysomelid beetles. III. The effects of killing parts of the eggs of Leptinotarsa decemlineata. Biol. Bull. 20: 237-251.
Illmensee, K. and Mahowald, A. P. 1974. Transplantation of posterior polar plasm in Drosophila. Induction of germ cells at the anterior pole of the egg. Proc. Natl. Acad. Sci. USA 71: 1016-1020.
Illmensee, K., Mahowald, A. P. and Loomis, M. R. 1976. The ontogeny of germ plasm during oogenesis in Drosophila. Dev. Biol. 49: 40-65.
Kunz, W., Trepte, H. H. and Bier, K. 1970. On the function of the germ line chromosomes in the oogenesis of Wachtiella persiariae (Cecidomyiidae). Chromosoma 30: 180-192.
Okada, M., Kleinman, I. A. and Schneiderman, H. A. 1974. Restoration of fertility in sterilized Drosophila eggs by transplantation of polar cytoplasm. Dev. Biol. 37: 43-54.